The present invention relates to improving the performance of polyimide membranes and particularly aromatic polyimide membranes as used for the separation of mixtures of gases. Aromatic polyimide gas separation membranes are known in the art. Such membranes generally fall within one of two categories. One such category incorporates membranes with high relative selectivities for one gas over another gas permeating through the membrane. Such membranes, however, suffer from having low gas permeation rates. This category of aromatic polyimide gas separation membranes is exemplified by U.S. Pat. No. 4,113,268; U.K. No. 1,435,151; U.S. Pat. No. 4,378,400; and EPO No. 113,574. Another category of polyimide gas separation membranes have much higher gas permeation rates, but suffer from correspondingly lower relative gas selectivities. Such materials are exemplified with U.S. Re. 30,351; U.S. Pat. No. 3,822,202; U.S. Pat. No. 3,899,309; U.S. Pat. No. 4,705,540; U.S. 4,717,394.
Many of the polyimides disclosed in the above-cited references are soluble in typical laboratory solvents. While this property is very desirable for ease of fabrication of gas separation membranes, it is undesirable for such gas separation membranes with respect to environmental conditions encountered during use thereof.
It is desirable to have polyimide gas separation membranes which exhibit high gas permeation rates while maintaining high relative gas selectivities. Further, it is desirable for such materials to be easily fabricated into appropriate membrane structures while maintaining excellent end use environmental resistance. One class of polyimide materials which incorporate many of these desirable characteristics are described in U.S. Pat. No. 4,717,393. These materials suffer from the inclusion of a complicated photochemical step within the process.